Control system for continuous-casting drive unit

ABSTRACT

A system is disclosed in which molten metal flows from a reservoir into a cooled forming die from which externallysolidified lengths of metal are drawn under precision control to produce a continuous integral shape. The drawing motion is critical in that incremental lengths are drawn during discrete time intervals between which the continuous integral shape is moved a short distance in the opposite direction to preserve continuity and compensate for shrinkage occurring during metal solidification. The disclosed embodiment of the system involves the development of an electrical control signal that is somewhat locked to preserve a predetermined relationship between the opposed critical motions.

United States Patent [191 Kuttner 1 CONTROL SYSTEM FOR CONTINUOUS-CASTING DRIVE UNIT [75] lnventor: Ralph Kuttner, Fullerton, Calif.

[73] Assignee: Stoody Company, Santa Fe Springs,

Calif.

[22] Filed: July 23, 1973 [2]] Appl. No.: 381,964

[52] US. Cl. 164/154; 164/260; 164/282 [51] Int. Cl. B22D 11/08; B22D 11/12 [58] Field of Search 164/4, 82, 154, 260, 282

[56] References Cited UNITED STATES PATENTS 2.8l5.55l l2/l957 Hcssenberg et al 164/83 3,669,176 6/1972 Krall et al. l64/4 Primary E.\'aminerR. Spencer Annear Alturney, Agent, or- F irmNilsson, Robbins, Bissell, Dalgarn & Berliner POW1Q SOURCE e o/P120044 [451 Sept. 30, 1975 [5 7 ABSTRACT A system is disclosed in which molten metal flows from a reservoir into a cooled forming die from which externally-solidified lengths of metal are drawn under precision control to produce a continuous integral shape. The drawing motion is critical in that incremental lengths are drawn during discrete time intervals between which the continuous integral shape is moved a short distance in the opposite direction to preserve continuity and compensate for shrinkage occurring during metal solidification. The disclosed embodiment of the system involves the development of an electrical control signal that is somewhat locked to preserve a predetermined relationship between the opposed critical motions.

4 Claims, 6 Drawing Figures \STROKE CONTROL SYSTEM FOR CONTINUOUS-CASTING DRIVE UNIT BACKGROUND AND SUMMARY oF THE INVENTION In general, difficulties encountered =in' continuouscasting processes include various production failures as by the billet freezing in the mold, breaking off, or the contents of the mold becoming molten, to result in an external flow of metal. In spite of such failures, continuous-casting processes have attained airelatively advanced state with regard to certain metals, e.g. alumiand copper. However, efforts to continuously cast certain other metals (for example high-temperature metals) particularly in lengths of relatively-small sectional size, have continued to present significant problems.

In general, the present invention relates'to the discovery that the continuous casting of certain metals involves precise control of the liquid phase of the metal within the die structure. As used herein, the term continuous casting relates to a continuous or integral product. Specifically, the system of the present invention is directed to a precise control system for drawing or moving the cast shape from the die during intermittent forward strokes which are time-separated by lesser return strokes. The system develops an electrical signal for control so that the strokes are proportionallyv related, so as to precisely compensate for the shrinkage attendant metal solidification.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings, which constitute a part of this specification, an exemplary embodiment exhibiting various objectives and features hereof is set forth, specifically:

FIG. 1 is a diagrammatic view of a casting system embodying a control unit constructed in accordance with the present invention; I

FIG. 2 constitutes a series of fragmentary sectional views through the die portion of the system of FIG.'1 which are illustrative of the process of the present invention;

FIG. 3 is a graph which is referenced in explaining the process of the present invention; and

FIG. 4 is a block diagram of the drive unit in the system of FIG. 1.

DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENT As required, a detailed illustrative embodiment of the invention is disclosed herein which is representative of the best mode within the purview of present knowledge for that purpose. The embodiment exemplifies the invention which may, of course, be constructed in accordance with other elements and steps, some of which D. In the operation of the system, molten metal entering a casting form in the die structure D is externally solidified (adjacent the die walls) so that it may be drawn in a rod form from the die structure D by a withdrawal roller mechanism R. A drive unit U actuates the roller mechanism R in a manner so that the billet or rod B (or other desired linear casting form) is drawn from the die structure D in an intermittent-motion pattern. That is, a length of rod B which has been externally solidified in the die structure D is drawn in what might be termed a forward drawing or withdrawal stroke. Then following each withdrawal stroke (away from the die), the roller mechanism R is actuated by the unit U to accomplish a reverse movement or abbreviated return stroke of the rod B (toward the die) to compensate for shrinkage resulting from solidification of the metal and to preserve the rod B integral so as to accomplish a continuous shape. The motion pattern of the rod B is critical and is one of the elements of accomplishing the desired integral product.

- Considering the structure of FIG. 1 in somewhat greater detail, the furnace F contains a mass 12 of mol' ten metal for supplying the desired continuous rod B. Specifically, the mass 12 is contained in a crucible 14 of refractory material which is somewhat upright and defines a bottom outlet passage 16 from which molten metal flows to develop the rod B. An induction coil 18 is provided about the crucible l4 and is connected, as well known in the art, to an electrical power source 20. Of course, an external housing, as well known in the art, may be provided for the furnace F outside the coil 18.

Molten metal flowing from the reservoir mass 12 throughthe passage 16 is received in a casting form, specifically a generally-cylindrical casting cavity 22 of the die structure D. The cavity 22 is generally coextensive with the passage 16. As illustrated, the cavity 22 is concentric within a hollow annulus 24 through which coolant 26,e.g. water, is circulated, Specifically, an intake duct 28 receives coolant from a source (not shown) which coolant circulates through the annulus 24 and exhausts through an outlet duct 30. Recognizing.

that a variety of specific arrangements and materials may be employed, in one embodiment the annulus 24 has been formed of an alloy that is primarily copper. Accordingly, high-temperature capabilities exist along with good heat-transfer characteristics.

An analysis of the solidification of metal within the die structure D is offered below. However, as indicated above, it is to be understood that the rod B is drawn in a continuous integral form section-by-section from the die structure D by engagement with a roller mechanism R including a pair of pinch rollers 34 and 36. Of course, the rollers 34 and 36 may be variously supported for rotation and in that regard, the roller 34 is driven while the roller 36 functions as a back-up or idler. The roller 34 is connected (as indicated by a dashed line 38) to the drive unit U for accomplishing the desired reciproeating motion pattern. That is, the unit U actuates the roller 34 to draw a length of a rod B from the die structure D during a drawing stroke, then moves the rod B in a lesser reverse or return stroke to preserve integrity with the liquid phase and compensate for the shrinkage in the metal resulting from solidification.

The detailed operation of the system of FIG. 1 may now be best understood by assuming certain initial conditions and explaining the operating sequence that functions to initiate and maintain the continuouscasting process. Accordingly, assume initially that the molten mass 12 has not yet been placed in the crucible 14 (functioning as in a holding furnace) or, alternatively, that in an alternate structure a gate (not shown) is closed, inhibiting the flow of molten metal through the passage 16. In the latter configuration, sensors may be employed to provide control-signal information as a basis for opening a flow gate at a time when the molten mass attains a desired casting temperature.

Prior to the passage of molten metal into the casting cavity 22, the crucible 14 is preheated with a gas torch and a seed tube or hollow starting rod 40 is positioned in the die structure as illustrated in FIG. 2a. That is, one end 42 of the hollow rod 40 is positioned to lie well within the casting cavity 22 as illustrated. The passage 44 (through the rod 40) extends to an opposed end 46 which is received in a venturi vacuum pump 48. A radial bore 50 is provided contiguous to the end 42 of the rod 40 in the cavity 22 with the result that a stream of air is drawn through the passage 44. The effect of drawing the preheated air from the crucible 14 into cavity 22 and passage 44 is to preheat the annulus 24 and rod 40. This prevents water vapor produced by the combustion process of the gas-filled torch from forming a condensate within cavity 22 and rod 40.

When the crucible 14 and the mass of preheated metal has attained the desired casting temperature, flow is initiated by means of pouring the preheated metal into crucible 14 as indicated by 12 into cavity 22 (FIG. 2a). At that time, molten metal enters the passage 44 (substantially cooler) and a liquid-solid interface is developed, resulting in a configuration that simulates a stage in the casting process, deemed to be somewhat as illustrated in FIG. 2b. Certain of the molten metal flowing into the passage 44 solidifies and is welded to the starter rod 40 while the rod also provides an artificial metal skin 55 or solid exterior. The welded bond is facilitated by the preheat of the rod 40 by the flow of hot air from crucible 14 into passage 44 allowing sufficient molten metal to enter passage 44 prior to solidification to make a good bond. It is also noteworthy that as the molten metal enters the starter rod 40, gases are permitted to escape through the bore 50 then serving as a venting port rather than as during the preliminary period when the function was to prevent condensation from taking place inside the die and to preheat the rod 40.

Considering the subsequent operation of the system, a reoccurring cycle is performed to develop incremental lengths of the continuous integral rod B. Specifically, from the condition depicted in FIG. 2b, a length or incremental section 52 of externally-solidified metal is drawn from the casting cavity 22. Of course, the length 52 may vary in specific instances; however, it is generally less than the length of the casting cavity 22. The drawing stroke is indicated by an arrow 54 and is accomplished by the reciprocal-drive unit U (FIG. 1) and the coacting pinch rollers 34 and 36.

At the conclusion of the drive stroke, a newly-formed incremental section 52 of the rod has been drawn from the casting cavity 22. Thereafter, the return stroke occurs during which the section 52 is moved in a reverse direction which motion compensates for the shrinkage of solidification within the metal.

Although some fine-adjustment deviations may be desirable, the relationship of the drawing stroke to the return stroke is somewhat in the ratio of the volume of material drawn during a stroke, to the volume of the shrinkage for that volume of material upon solidification. Generally, the cross section of the rod B may be considered to be substantially uniform. Consequently, the volume of the incremental section 52 provided during each drawing stroke is related. to the length of the stroke. Therefore, the length of the return stroke is generally adjusted to coincide to the volumetric change of the casting metal attendant solidification.

In addition to the criticality of the length or distance covered by drawing and return strokes, motion patterns of the strokes are also significant. For example, the acceleration of the strokes are controlled in relation to the solidification characteristics of the casting metal as will now be considered with reference to FIG. 3.

As generally well known, the solidification of a volume of metal located in a cooling environment is neither instant nor uniform. For example, the volume might solidify as indicated by the curve 60 (FIG. 3) which is plotted on a time base. Initially, solidification may occur very rapidly as crystalline fragments are formed until the curve 60 reaches a knee 62 after which solidification occurs at a slower rate until the section of metal has completely solidified.

Generally, in accordance herewith it has been determined to be desirable to provide acceleration patterns in the stroke motions, which accommodate the characteristic cooling curve of the cast metal. Specifically, the acceleration of the forward stroke is well suited to coincide to the curve 60; however, the acceleration of the return stroke is suited to the inverse curve 64 as indicated. Within the purview of these considerations, if accelerations are too rapid, the continuous process may be interrupted by a break in the skin 55 (FIG. 2c) whereas if accelerations are not sufficiently rapid, metal may freeze in the die cavity 22 with the result that the next length of metal to be drawn will separate from the liquid phase. To avoid such failures, control parameters are provided in the system of FIG. 1 by the reciprocal-stroke drive unit U, as will now be considered in detail with reference to FIG. 4.

Generally, the drive unit is electro-mechanical in that an electrical signal is developed which commands rotation of the roller 34 (FIG. 1). With some lag, rotation of the roller 34 translates into linear displacement of the rod B. Consequently, the developed electrical signal essentially commands an identical linear motion of the rod B.

The basic operation cycle for the formation of a length or section 52 of the rod B is established by a variable-cycle multivibrator 66. Recognizing that the periods of the drawing stroke and the return stroke may vary widely, the multivibrator 66 may be set (manually for example) to provide an output having a desired waveform 68 including a positive-going pulse 70 and a negative-going pulse 72. The positive-going pulse 70 is provided as an initial approximation to the interval of the drawing stroke while the negative-going pulse 72 is to approximate the interval of the return stroke. Thus, two control-signal components are initially approximated. Generally, various forms of multivibrators are well known in the prior art which may be adjusted to provide waveforms with any of a broad range of time relationships and durations for the pulses 70 and 72.

The output from the multivibrator 66 is provided to a polarity splitter 74. Functionally, the splitter 74 separates positivegoing pulses 70 from negative-going pulses 72 to provide the signal components as dual outputs to conductors or lines 76 and 78. Generally, the splitter 74 may comprise any of a variety of dual-output amplifiers as well known in the electronics arts and referenced to provide stable output signals.

The positive-going pulses 70 (appearing on a line 76) are developed to define the interval and nature of a drawing stroke while the negative-going pulses 72 (appearing on line 78) are developed to define the interval and nature of a return stroke.'As will now be considered in detail, the pulses 70 and 72 are shaped to desired forms to accomplish the specific motion patterns deemed appropriate to the process.

The line 76 is directly connected to an edge shaper 80. The line 78 is connected to a monostable multivibrator 77 and to a mixer 79, the output of which is applied to edge shaper 82. The shaper circuits may take a wide variety of forms including simple filters and complex operational amplifiers (as may have an integrating characteristic) and memory-type curve generators. The edge shaper 80 processes the pulses 70 carried by the line 76 to produce pulses 84 having sloped leading edges in accordance with the considerations set forth above with reference to Fig. 3. The pulses 72 are developed to control the return stroke and in view of mechanical response delays that are characteristic of certain apparatus, it has been determined to be advisable to provide a steep spike at the leading edge that initiates the motion. Accordingly, the multivibrator 77 is triggered by the leading edge of the pulses 72 to provide negative spikes 83 which are combined with the pulses 72 by the mixer 79 to produce combination pulses 85, somewhat idealized in representation. The pulses 85 then are shaped by the shaper 82 to develop the pulses 86 which are applied through a divider 90 to the mixer 88 along with the pulses 84.

The mixer 88 combines the signal components in scaled relationship. Specifically, the pulses 84 are combined with a divided or scaled-amplitude form of the pulses 86. The divider 90 is connected to receive the pulses 72 to control the interval during which the di vider 90 is operative. Generally, the divider 90 may .comprise any of a variety of conditionally-operative analog divider circuits for providing an output with a form essentially of the pulse 86; however, scaled in accordance with the amplitude of a signal received through a line 92. As a consequence, the output of the mixer 88 is the waveform 94 including a positive half cycle 96 (indicating the drawing stroke) and a negative half cycle 98 (indicating the return stroke).

It is significant for these control-signal components that the area defined by the negative half cycle 98 be scaled in a relatively constant relationship to the area defined by the positive half cycle 96. That is, the area defined by the positive half cycle 96 defines the volume of rod or length of material drawn during a forward or drawing stroke. To compensate for shrinkage and maintain an integral continuous shape, as indicated above, it is important that a return stroke be provided which is related to the characteristic of the material and the length of the drawing stroke (assuming a uniform cross section).

In accordance with the operation of the system of FIG. 4, the area defined by the positive half cycle 96 is integrated to provide a value for scaling the negative half cycle 98. Specifically, determining the area defined by the positive half cycle 96 is one element in establishing a constantly-related area to be defined by the negative half cycle 98. As indicated above, the interval of the negative half cycle 98 is known as is the desired constant relationship between the two half cycles. Accordingly, the known area of the positive half cycle 96 may be directly scaled by the divider to attain the desired return motion which translates directly into a volumetric relationship. That is, in view of the uniform crosssection of the casting, the waveform 94 (plotted on a time base) is employed as a speed control to maintain a desired volumetric relationship.

Control for the divider 90 is provided by an integrator 100 functioning to integrate the pulses 84 over the timed interval of such pulses. Generally, the integrator IOOmay comprise an operational amplifier connected to accomplish the integrating function as well known in the prior art. The integrated values provided from the integrator 100 are registered in a store 102 which may comprise any of a variety of analog registers. From the store 102, an analog signal is provided to the divider 90 to accomplish and preserve the desired constant relationship between the area defined under the positive half cycle 96 and that defined under the negative half cycle 98. Consequently, the signal indicated by the waveform 94 may be employed to control the speed of the uniform-cross section casting and thereby establish the desired volumetric relationship.

The output electrical signal from the mixer 88 (waveform 94) is applied to an electro-hydraulic velocity controller 106. The controller 106 is connected to the roller 34 (FIG. 1) and, accordingly, the speed of the roller 34 is controlled in accordance with the electrical signal as represented by the waveform 94. The electrohydraulic velocity controller 106 may take a variety of forms of structures well known in the prior art. Specifically, velocity-control systems are described beginning on page of a book entitled ELECTROI-IYDRAU- LIC SERVOMECl-IANISMS by Allen C. Morse, published in 1963 by McGraw-Hill Book Company.

From the above explanation it may be seen that the present system affords precise control in drawing a continuous casting, length-by-length to accomplish an elongated form which, is desired, may be reduced to convenient sizes. In operating embodiments, continuous lengths of cylindrical rod have been produced of high-temperature metal, in small sizes, for example to three-sixteenths diameter. Such product has been found to contain few contaminants and, accordingly, may be employed directly for welding without further processing. Of course, specific setups and adjustments will be required for specific castings and additionally, it will be appreciated that a variety of different component parts may be employed in the system. As a consequence, it is to be understood that the scope hereof shall be in accordance with the claims as set forth below. Q

What .is claimed is: V

l. A control system, as for use with a continuous casting apparatus wherein molten metal is provided in a casting cavity to be cooled in exiting therefrom for movement in an externally-solidified continuous-length means for shaping said first electrical pulse in relation to a solidification characteristic of said molten metal to provide a first control signal component;

means for applying said first control signal component to said actuating means as said control signal to move said form away from said casting cavity;

means for providing a second control signal component of opposed polarity to said first control signal component, and proportionately related to said first control signal component on the basis of time and amplitude; and

means for applying said second control signal component to said actuating means along with said first control signal component, to move said form controller. 

1. A control system, as for use with a continuous casting apparatus wherein molten metal is provided in a casting cavity to be cooled in exiting therefrom for movement in an externallysolidified continuous-length form from said casting cavity, comprising: bidirectional actuating means including means for engaging said continuous-length form, for moving said form both toward and away from said casting cavity for the progressive development thereof, said actuating means being responsive to variations of a control signal waveform, with respect to a reference level indicative of a stationary state; a variable signal generator means for providing a first electrical pulse; means for shaping said first electrical pulse in relation to a solidification characteristic of said molten metal to provide a first control signal component; means for applying said first control signal component to said actuating means as said control signal to move said form away from said casting cavity; means for providing a second control signal component of opposed polarity to said first control signal component, and proportionately related to said first control signal component on the basis of time and amplitude; and means for applying said second control signal component to said actuating means along with said first control signal component, to move said form toward said casting cavity.
 2. A system according to claim 1 wherein said means for providing a second control signal component includes means for integrating said first control signal component.
 3. A system according to claim 1 wherein said means for providing a second control signal component includes further means for shaping said first electrical pulse in relation to a solidification characteristic of said molten metal.
 4. A control system according to claim 1 wherein said actuator means comprises an electro-hydraulic velocity controller. 